Without limiting the scope of the present invention, the background of the invention is described with reference to polymer blends, in particular, polymer blends useful and particularly adapted for the production of carpets and rugs. Due to widespread availability and economic factors, polypropylene is a material particularly suitable for use in carpet manufacture. However, polypropylene fiber, texturized and woven into carpets or rugs, typically does not possess all of the most desired properties for use as a material for carpet fibers.
In particular, polypropylene fibers, texturized and woven into carpets and rugs, typically do not possess the high resiliency characteristics of nylon or polyester fibers because of the relatively low glass transition temperature, (T.sub.g), of the polypropylenes utilized to produce such fibers. Since the glass transition temperature of the polypropylenes utilized to produce carpet fibers is typically below room temperature, the molecular mobility of the polypropylene molecules is such that the fibers deform, without substantial recovery, in the direction of the applied load. Consequently the use of polypropylene fibers in the manufacture of carpets and rugs has been limited.
Typically, the glass transition temperature of commercially available polypropylene is in the range of 0.degree. C. Thus, at room temperature, portions of polypropylene molecules, especially amorphous segments, retain mobility over a significant range of the segment length. Consequently, if a load is placed upon a carpet produced from polypropylene fibers at room temperature, the polypropylene molecules tend to relax in the direction of stress and retain the imposed deformation after the load is removed. One way of minimizing non-reversible fiber deformation is to raise the glass transition temperature of the polymer. If the glass transition temperature of the material can be increased, the effect of segmental molecular deformations can be reduced or significantly reduced. Thus, if the glass transition temperature of the polymer is increased, the fiber will be more likely to recover from the deformation after the load is removed.
One way of increasing the glass transition temperature of a polymer is to increase the size of the pendant groups on the polymer backbone. For example, in the case of polypropylene, increasing the glass transition temperature could theoretically be accomplished by replacing some of the pendant methyl groups with larger pendant groups. One way of achieving this result would be to copolymerize propylene with bulkier olefins. However, as a practical matter, copolymerization with a higher olefin is not necessarily a commercially feasible solution due to numerous factors, including loss of productivity due to decreased polymerization rates and a decrease in the control over the physical properties of the resulting copolymer.
Another approach to increasing the glass transition temperature of polypropylene is to physically mix another polymer with a higher glass transition temperature with the polypropylene. The added polymer must, however, be physically compatible with polypropylene and economically feasible to use in the desired application. Because physical mixtures of different polymers adhere to each other via secondary bonding forces, the chemical composition, crystal structure, morphology and molecular weight all impact on the compatibility of the polymers. Finally, the polymer blend should be adaptable to processing, utilizing existing process equipment and conditions.
One article, Gupta et al., "Processability and properties of yarns made from polypropylene containing small amounts of polystyrene", J. Appl. Polm. Sci. 60, 1952 (1996), reports higher drawability and elongation with the addition of polystyrene to polypropylene. However, the maximum draw ratio reported in the Gupta article was 5.1 at a wind up speed of 750 (m/sec). Additionally, the reported tenacity of the resultant fiber was relatively low and the reported thermal shrinkage of the fibers was relatively high.
Thus, there exists a need for an improved blended polypropylene for use in the manufacture of fibers, and in particular, polypropylene fibers that exhibit improved resiliency, tenacity, processability and thermal resistance.